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.Dd December 1, 2022
.Dt ISATTY 3C
.Os
.Sh NAME
.Nm isatty
.Nd test for a terminal device
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fo isatty
.Fa "int filedes"
.Fc
.Sh DESCRIPTION
The
.Fn isatty
function tests whether
.Fa fildes ,
an open file descriptor, is associated with a terminal device.
.Sh RETURN VALUES
The
.Fn isatty
function returns
.Sy 1
if
.Fa fildes
is associated with a terminal; otherwise it returns
.Sy 0
and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn isatty
function may fail if:
.Bl -tag -width Er
.It Er EBADF
The
.Fa fildes
argument is not a valid open file descriptor.
.It Er ENOTTY
The
.Fa fildes
argument is not associated with a terminal.
.El
.Sh USAGE
The
.Fn isatty
function does not necessarily indicate that a human being is available for
interaction via
.Fa fildes .
It is quite possible that non-terminal devices are connected to the
communications line.
.Pp
There are a few portability concerns to be aware of.
POSIX does not strictly require that the
.Fn isatty
function actually set and update
.Va errno .
The only portable and reliable thing to do is to check the return value.
Older versions of this implementation did not set
.Va errno
and erroneously preserved
.Va errno
when the
.Fn isatty
function returned 0.
In cases where true portability is required, one should set
.Va errno
to zero before calling this function to potentially allow for and distinguish
this case.
.Sh INTERFACE STABILITY
.Sy Committed
.Sh MT-LEVEL
.Sy MT-Safe
.Sh SEE ALSO
.Xr ttyname 3C ,
.Xr attributes 7 ,
.Xr standards 7
